US3216838A - Silica refractories - Google Patents

Description

United States Patent 3,216,838 SILICA REFRACTORIES Raymond E. Birch, Donald F. King, and Ben Davies,

Pittsburgh, Pa., assignors to Harbison-Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Mar. 27, 1964, Ser. No. 355,431

4 Claims. (Cl. 106-69) The present invention relates to improved silica refractory shapes and brick for use in such applications as byproduct coke ovens and the like.

By-product coke ovens are long narrow chambers lined with silica brick and usually joined together in batteries of up to 100 or more ovens. The ovens are separated from each other by silica brick walls which also enclose the heating flues that supply the heat for coking the coal. By this means, the combustion gases from the heating fines and the gaseous products of carbonization are kept separate at all times.

In the operation of coke ovens, the ovens are readied for charging by setting the doors in place. The coal is then charged into the coking chambers and the firing cycle begun. Coking is normally completed in 14 to 18 hours depending on the width of the oven and the temperature employed. When the coking cycle is completed, the doors are removed and the coke is pushed out of the oven by an electrically driven ram into the quenching car. The quenching car transfers the hot coke to the quenching station where the coke is cooled by spraying with water. After the coke is pushed out of the oven, the doors are replaced and the oven prepared for charging again.

The principal refractory material used in the construction of by-product coke ovens is silica brick. Silica brick have two inherent qualities which recommend them for such service. The more important property is their volume stability at the operating temperatures of the coke oven. The reversible thermal expansion of silica brick is essentially complete below about 1060 F. Thus, the continuing high temperature expansion experienced with other types of refractory brick and which must be allowed for instructures is absent when silica brick form the construction. The other unique property of silica brick is their ability to withstand high loads and to remain rigid up to within a few degrees of their actual melting point of 3140" F. For example, conventional first quality silica brick do not fail in the load test at p.s.i. until a temperature of about 2975 to 3025 F. is reached and superduty silica brick, as exemplified in United States Patent No. 2,351,204, wherein the total of alumina, titania, and alkalies does not exceed 0.5 weight percent, withstand the 25 lb. load up to about 3075" or even 3090 F.

In coke oven operation, it will be realized that the highest temperatures are encountered in the oven walls. These walls are also subjected to very rapid changes in temperature where the cold coal is charged into the hot oven. 'However, experience has shown that even though the wall temperature rarely drops below 1000 F. and, therefore, spalling due to thermal shock is not a serious problem, it is a factor which must be considered. The normal top temperature recorded in the oven walls is about 2800 F. so the strength of the silica is not jeopardized.

The walls of the coke oven are subjected to severe abrasive action during charging and when the coke is pushed out into the quenching car. The oven chamber has even been tapered to reduce the abrasive action against the wall 'as much as possible during pushing. Therefore, the ability of the wall brick to elfectively resist the abrasive efiects of the coke bears a direct relation to the life ice of a coke oven battery. As explained above, the heat for coking is supplied from fines which are enclosed in the silica walls. Thus, the thermal conductivity of the wall material is an important factor in providing for the transfer of heat from these lines to the coke. It can easily be understood that a material possessing a high thermal conductivity would enable a more economical operation of the coke oven than one of lower conductivity.

It is therefore an object of the present invention to provide silica brick with increased spalling resistance, increased resistance to abrasion, greater density, and a higher thermal conductivity than is presently experienced, as well as to provide improved articles such as coke oven walls and the like constructed of the brick.

Other objects of the invention will become apparent hereinafter.

Briefly, in accordance with the present invention, there is provided a fired silica refractory brick formed from a batch consisting essentially of, by weight, from 1 to 5% total of at least one member of the group consisting of calcium oxide and magnesium oxide, 2 to 5% of titanium dioxide and the remainder silica rock or quartzite. For superduty type brick, chemical analysis of the batch will show not over 0.5% total of alumina (A1 0 titania (TiO and the alkalies (Na O and K 0). For conventional silica brick, the materials may range, in the aggregate, up to about 0.8 to 1.5%. The composition is further characterized, in the instance of superduty quality, in that chemical analysis will show that the one or more members of the group of calcium and magnesium oxides are present in a total amount of at least 3.3 times the silica rock content of alumina, titania, and alkalies. The lime (CaO) and magnesia contents are supplied by that added as bond, usually the commercial hydrates. The silica rock or quartzite used in the compositions may be any one of the types commonly used in making silica brick, with the purity level being determined by whether regular or superduty brick are to be made. As mined, the silica mineral may consist of quartzite in massive form or an agglomerated quartzite pebble. Other forms of silica rock used for silica brick manufacture are also suitable.

Titanium dioxide (TiO used in the invention is added as a separate batch ingredient (i.e., it is in addition to the TiO which appears as an impurity in the silica rock). We prefer a pigment grade form. Such titanium dioxide is usually a precipitate or condensate of a fluid. Preferably the oxide is finely divided to the extent that substantially all of the particles are five microns or less in size. This fineness is important in obtaining a sul'iiciently dense brick and may be considered critical for this purpose. It is incorporated in the batch in any manner which brings about a thorough dispersion of the titanium dioxide through the batch. About 2 to 5%, based on the resulting batch,'of the titanium dioxide is used with about 3 to 4% being the preferred range.

The lime used for bonding will ordinarily be commercial hydrated lime. Dolomitic lime (CaO-MgO) is also usable and will likewise ordinarily be added as the hydrate. When magnesia (MgO) is used alone, it will be preferable to use readily hydratable light burned magnesia (caustic magnesia). There is nothing in these practices which is not well known in the art of silica brick manufacture. The lime or magnesia added to the batch is spoken of as the bond, since it is available both as a bond in the fired brick and also provides strength in the unfired brick. In silica brick manufacture, lime commonly is used in amounts of 1 to 5% (on the basis of Tyler mesh: Percent -6 +10 10 1O +28 30 2s +65 16 65 44 About by weight, of water was added as was about 1% of concentrated sulfite liquor, a temporary bonding agent. The batch was pressed into brick, measuring 9 x 4 /2 x 3", at about 8000 p.s.i. The brick were removed from the press and dried for about 24 hours at 250 F. The dried brick were fired in a tunnel kiln for 5 days, reaching a top temperature of 2700 F.

The quartzite used in these examples analyzed about 99.5% SiO with Al O +Fe O +TiO +alkalies, the remainder. The added titanium dioxide was pigment grade, as supplied by the National Lead Company, and was all less than 5 microns in particle size. Small amounts of ---150 mesh iron pyrite cinder and alumina were added to increase the strength of the brick (with some loss of refractoriness). Any aluminous clay and iron ore or iron oxide is similarly usable. The batch components and the data obtained on the resulting brick are shown in Table I.

Silica spalling test (SEVERITY OF CRACKING WHEN HEATED TO 1500 F. AT A GIVEN RATE) None.

300 FJhr. heating rate Considerable Moderate.

500 FJhr. heating rate Severe The thermal conductivity measurements on the mixes were also taken and are indicated in Table II.

TABLE II Thermal conductivity Mean temperature, F.

It can be observed that the addition of titanium dioxide improved each of the properties tested, with the exception of cold crushing strength which was substantially the same. The strength of the brick, as evidenced by the modulus of rupture, was greatly improved.

It is well known in the refractory art that the transverse strength of refractory bodies bears a fairly direct relation to abrasion resistance. Modulus of rupture is a standard test in refractory studies. It is determined with simple apparatus, exhibits a good degree of precision, and provides an excellent measure of bonding strength. Therefore, its determination is often made in lieu of abrasion testing which requires much more elaborate equipment. Accordingly, in the present instance, the increase in strength shown for batch No. 2 is indicative of great improvement in abrasion resistance.

The strength of silica brick has always been a problem for the refractories manufacturer. Much work has been done in past years to develop stronger bonds without seriously affecting the inherent properties of brick. A low modulus of rupture is very likely to result in broken corners and edges during handling and shipping. Small variations in batching, sizing, mixing or pressing conventional mixes can result in brick which are too weak to be used satisfactorily. The almost twofold increase in strength provided by the addition of 3% titanium dioxide is therefore a great improvement.

The beneficial effect on resistance to thermal spalling by the addition of titanium dioxide is also apparent from the above Table I. Silica brick must be heated slowly up to about 1500 F. because most of their thermal expansion occurs below this temperature and they are extremely susceptible to spalling below this figure. It can be observed that batch No. 2 showed no cracking when heated at 400 F. per hour whereas the conventional superduty brick cracked considerably. Also, when heated at 500 F. per hour, the brick with the titanium dioxide addition cracked only moderately whereas the conventional brick was rendered useless.

The increase in thermal conductivity resulting from the addition of titanium dioxide plainly is apparent from the results indicated in Table II.

Superduty coke oven brick generally have a lower alumina (i.e., less than 0.3%) and iron oxide content than conventional silica brick. An exemplary superduty brick consists of 96.5% quartzite and 3.5% hydrated lime. Accordingly, the overall strength of the brick will be lower than conventional brick. This higher strength is particularly necessary to permit safe handling and shipping of special coke oven shapes and reduce breakage in transit. The higher alumina and iron oxide content may be obtained by adding minor amounts of alumina and iron ore or iron oxide as was done in the examples of Table I or by using less pure quartzite containing iron oxide and alumina. However, the addition of from 2 to 5% titanium dioxide to a batch used to make Superduty-type silica brick will increase each of the properties tested above to about the same proportion as was realized with conventional brick. Accordingly, it can be appreciated that where the choice of silica brick type is dependent on strength,, Superduty-type brick (i.e. containing no more than 0.5% total of A1 0 TiO and alkalie-s initially) containing additional titanium dioxide may be employed.

As can be seen from the foregoing, the addition of titanium dioxide to silica brick provides brick having increased resistance to abrasion, increased resistance to thermal shock, great-er density and greater thermal conductivity. However, additions of titanium dioxide have a somewhat deleterious effect on the refractoriness of the brick and, therefore, more than five percent cannot be safely tolerated.

In the foregoing discussion and description, all percentages are by weight unless otherwise stated. Similarly, the brick were prepared by conventional techniques and the property data obtained by tests that are standard in the refractory arts.

Having thus described the invention in detail and with sufficient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters Patent is set forth in the following claims.

We claim:

1. In a fired silica refractory brick formed from a refractory size graded brickmaking batch consisting essentially of, by weight, from about 1 to 5% total of at least one member of the group consisting of calcium oxide and magnesium oxide, from about 90 to 98% of silica rock, no more than 1.5% total weight percent of A1 0 TiO and alkalies in said silica rock, the improvement comprising from about 2 to 5% of TiO as a separate batch constituent, said brick being characterized by resistance to abrasion, resistance to thermal shock, increased density and good thermal conductivity.

2. In the fired silica refractory brick of claim 1, said batch containing titania in the range of about 3 to 4%.

3. In the fired silica refractory brick of claim 1, said batch containing up to about 0.3% of a material selected from the group consisting of iron oxide and iron ore, and up to about 0.5% of a material selected from the group consisting of alumina and aluminous clay.

4. In the fired silica refractory brick of claim 2, said titania being substantially all 5 microns or less.

References Cited by the Examiner UNITED STATES PATENTS 2,662,021 12/53 Keltz 10669 Bricks, trans., Ceram. Soc., 18, Pt. (11), 481-596, (1919). (Abstracted in J. Am. Cer. Soc., Vol. 3, 1920 page 335.)

5 (Scientific Library TP 785 A. 62.)

References Cited by the Applicant UNITED STATES PATENTS 10 Re. 25,572 5/64 McCreight.

2,351,204 6/44 Harvey et al. 2,494,276 1/50 Austin et al. 2,662,021 12/ 53 Keltz. 3,082,105 3/63 Osborn. 3,157,522 11/64 Stookey.

FOREIGN PATENTS 9/59 Great Britain.

OTHER REFERENCES Factors Influencing the Properties of Silica Bricks, Part 1, Scott, American Ceramic Society Journal, Vol. 3, page 335.

TOBIAS E. LEVOW, Primary Examiner.

Claims (1)

1. IN A FIRED SILICA REFRACTORY BRICK FORMED FROM A REFRACTORY SIZE GRADED BRICKMAKING BATCH CONSISTING ESSENTIALLY OF, BY WEIGHT, FROMABOUT 1 TO 5% TOTAL OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF CALCIUM OXIDE AND MAGNESIUM OXIDE, FROMABOUT 90 TO 98% OF SILICA ROCK, NO MORE THAN 1.5% TOTAL WEIGHT PERCENT OF AL2O3, TIO2, AND ALKALIES IN SAID SILICA ROCK, THE IMPROVEMENT COMPRISING FROM ABOUT 2 TO 5% OF TIO2 AS A SEPARATE BATCH CONSTITUENT, SAID BRICK BEING CHARACTERIZED BY RESISTANCE TO ABRASION, RESISTANCE TO THERMALSHOCK INCREASED DENSITY AND GOOD THERMAL CONDUCTIVITY.